In many aircraft and other airborne systems, AC power is generated from a brushless synchronous generator. FIG. 1 illustrates a conventional configuration of a brushless synchronous generator excitation power arrangement connected to a generator control unit.
A brushless synchronous generator 104 will typically include three types of alternating current (AC) synchronous generators: a main generator 106; an exciter 108; and a permanent magnetic generator (PMG) 110 connected to the same rotating shaft 112. The exciter 108 is used to achieve brushless excitation and the PMG 110 is the power source providing power to the exciter 108 and a generator control unit (GCU) 102.
As shown in FIG. 1, the GCU 102 contains a three phase rectifier 114, a generator control relay 120, a power supply 116, a field switch 119 and a field switch driver 118. The power supply 116 is provided with backup power 123. A free wheeling diode 117 is connected between line 113 and line 115. Line 113 connects an output line of the three phase rectifier 114 with one end of the stator winding of the exciter 108. Line 115 connects the field switch 119 to the other end of the stator winding of the exciter 108. The three phase rectifier 114 is electrically coupled to the PMG 112 and the exciter 108.
In FIG. 1, the PMG power is input to the GCU 102. In the GCU 112, the three-phase PMG AC power is scaled and rectified in the three phase rectifier 114. When the field switch 119 is ON, the free wheeling diode 117 is reverse biased and is in a blocking state. When the field switch 119 is OFF, the free wheeling diode is forced on and the diode creates a free-wheeling path for excitation energy from the exciter 108 stator coil. The rectified PMG power is fed to the exciter stator coil to excite the generator. Since the exciter power is only rectified before being input to the exciter 108, its voltage level varies with generator speed. When the generator control relay 120 closes, a connection from the three-phase rectifier 114 is made and exciter power flows to a coil of the exciter 108.
In an AC generator, the generated voltage varies with the rotating speed of the generator. In constant frequency electric power systems where the generator speed is almost fixed, the output voltage from the PMG changes very little. Therefore, after applying rectification and scaling, the PMG power can be controlled to energize the exciter.
However, in recent years, variable frequency (VF) power systems have gained in popularity and applications. In a VF power system, the non-constant PMG voltage can cause problems. In a VF power system, the speed of the generator varies with the engine speed, thereby causing the PMG output voltage to randomly change or vary over a wide range.
In a conventional configuration, the supply voltage to the exciter will vary in the same manner as the generator speed varies. A generator exciter requires higher field or excitation currents when the generator speed is low or the load on the generator is high. Conversely, a generator exciter requires lower field or excitation currents when the generator speed is high or its load is low. Because the PMG voltage is proportional to the generator speed, a high exciter power supply voltage can cause control difficulty. For example, the generator may not work properly or at all under certain operating conditions, such as high-end speed and light load condition.
Therefore, it is desirable to have regulated exciter power for a brushless synchronous generator that solves the aforementioned problems.